As a thermoelectric device generates a voltage if the opposite ends thereof are maintained at different temperatures, the same is utilized for thermoelectric power generation, and conversely, if electric current is caused to flow therethrough, an exothermic reaction occurs at one end thereof while an endothermic reaction occurs at the other end thereof. Accordingly, the same is also utilized in a cooling apparatus, and so forth, making use of an endothermic phenomenon. Because such a thermoelectric device as described above is simple in construction, and has an advantage over other electric power generators in implementation of miniaturization, and so forth, hopes run high that the same will be applied to portable electronic equipment such as an electronic wrist watch.
The thermoelectric device is made up of a plurality of thermocouples arranged in series, each composed of a p-type semiconductor thermoelectric material and an n-type semiconductor thermoelectric material. The construction of such a common type thermoelectric device as above is described with reference to FIG. 19.
A thermoelectric device 10 shown in FIG. 19 has a thermoelectric device block 11 wherein p-type thermoelectric semiconductors 1 and n-type thermoelectric semiconductors 1 are alternately disposed with an insulation layer 4 made of epoxy resin, interposed therebetween, respectively. A conductive film 3 provided on an end face of the respective thermoelectric semiconductors 1, on opposite sides thereof, is connected with a wiring electrode 6 made of copper or gold, provided on substrates 7, respectively, through the intermediary of respective connection layers 5, thereby rendering the thermoelectric device block 11 electrically continuous with the substrates 7, and connecting the respective thermoelectric semiconductors 1 with each other in series.
Prior to connecting the thermoelectric device 10 with the substrates 7, the conductive film 3 is formed on the end face of the respective thermoelectric semiconductors 1, on the opposite sides thereof, to be connected with the respective wiring electrodes 6. This is necessary for the following reasons.
The connection layers 5 are provided in order to ensure electrical continuity between the respective thermoelectric semiconductors 1 and the respective wiring electrodes 6, however, if the connection layers 5 are formed of solder, tin contained therein is diffused into the respective thermoelectric semiconductors 1, causing deterioration in performance of the thermoelectric device 10. Accordingly, it is necessary to form the conductive films 3 for elimination of such a risk and to ensure wettability of solder. Further, in the case of forming the connection layers 5 from a conductive adhesive, it is necessary to form the conductive films 3 having a low contact resistance against the conductive adhesive because of a large contact resistance between the respective thermoelectric semiconductors 1 and the conductive adhesive.
In the case of forming a metallic film on a thermoelectric semiconductor, serving as a conductive film, plating is generally adopted. In applying plating, an electroless plating method using a self-catalyzing type electroless plating bath is advantageous in terms of productivity. It is not possible, however, to apply electroless plating to a thermoelectric semiconductor composed of an intermetallic compound of a bismuth-tellurium base or an antimony-tellurium base.
For this reason, in the case of forming a conductive film on the surface of material such as a thermoelectric semiconductor to which it is not possible to apply electroless plating, it has been a normal practice to apply electroplating thereto.
For the formation of the conductive film on the surface of the thermoelectric semiconductor by electroplating, however, electric power needs to be supplied to the thermoelectric semiconductor, which has caused a problem in that the thickness of a plating film formed becomes thinner according as a distance from the point of power supply increases due to a voltage drop caused by a resistance value of the thermoelectric semiconductor. This has resulted in fluctuation in the thickness of the conductive film made up of the plating film, thereby impairing an effect of preventing diffusion of tin contained in solder, and adversely affecting wettability of solder.
In JP11-186619, a method of applying electroless plating by providing a thermoelectric semiconductor with a catalyst, such as platinum, palladium, and so forth, is disclosed as a method of forming a conductive film on a constituent material to which it is not possible to apply electroless plating.
This method, however, is a method whereby electroless plating is implemented by providing a catalyst as seed crystals, and is a method generally adopted for forming a conductive film on plastics. With the method described, there is eliminated the abovementioned problem of uneven thickness of the plating film formed by electroplating, but the following problem has been encountered.
That is, with this method, since adsorption of the catalyst to serve as the seed crystals occurs to parts other than the thermoelectric semiconductor, selectivity on regions where the conductive films are to be formed will be lost upon dipping the thermoelectric semiconductor in an electroless plating bath, causing a problem that the formation of the conductive films occurs to unnecessary regions as well, for example, on the surface of insulators.
Thus, there have so far existed not only a problem that it has not been possible to form the conductive films on the surface of a constituent material to which it is not possible to apply electroless plating, but also a problem that selectivity on the regions where the conductive films are to be formed has been lost even if the conductive films have been formed by electroless plating.
In particular, the thermoelectric device comprises thermoelectric semiconductors which are very small in size, and has sometimes a minuscule structure wherein the thermoelectric semiconductors are disposed at an interval between the adjacent thermoelectric semiconductors, in a range of several to several tens of μm. The more minuscule the structure of the thermoelectric device, the more difficult it becomes to form the conductive films selectively only on the thermoelectric semiconductors. It is therefore a major problem in the fabrication of the thermoelectric device to selectively form the conductive films by electroless plating.
The invention has been developed to solve those problems, and an object of the invention is to provide an electroless plating method whereby conductive films can be formed even on the surface of a constituent material to which it is not possible to apply electroless plating, and further, to selectively form the conductive films uniform in thickness on end faces of respective thermoelectric semiconductors formed of a constituent material to the surface of which it is not possible to apply electroless plating, thereby enhancing productivity and reliability of a thermoelectric device as fabricated.